The present invention relates to an LED drive device, an LED drive method and a lighting system. Particularly it relates to an LED drive device and an LED drive method for driving a plurality of series-connected LEDs, and a lighting system.
Lighting systems using a plurality of LEDs (Light Emitting Diodes) have recently been developed. A switching method and a linear control method have been used as control methods in the lighting systems. The former method uses a switching power supply circuit for supplying a constant current from an AC power supply to each LED. The latter method detect an AC voltage or a current flowing in LEDs and changes the number of switched-on LEDs to minimize power supply loss. Examples of conventional systems are illustrated in JP-A-2006-244848, JP-A-2008-59811 and JP-A-2006-147933
The switching method, however, has a problem in that noise occurs at the time of switching, and a problem that a large number of parts are required because the circuit configuration is rather complicated. Conventional linear control methods also has problems.
For example, in JP-A-2006-244848, when the alternating voltage value increases, switches are not turned off but the number of turned-on switches is increased. Accordingly, there is a problem that efficiency is very poor. In JP-A-2008-59811 and JP-A-2006-147933, the input voltage is monitored so that switching elements are switched in accordance with the input voltage. There is however a problem that this method cannot accept differences between forward voltages of LEDs. For example, when the forward voltages of LEDs mounted as products are 19V and 21V though the drive voltage of each LED is designed to be 20V, there is a possibility that the current flowing in the drive circuit may be interrupted.
For this reason, it is necessary to adjust the voltage at the time of shipping. There is a problem that the number of processes at the time of shipping increases.